"Trees in the City: Cooling, Carbon and the Complications"

Dr. Lucy Hutyra admits she's "a total tree-hugger." She's the Chair of the Department of Earth and Environment at Boston University, and a MacArthur Genius Grant recipient. She and her BU colleagues study the impact of trees on urban environments, particularly their effects on carbon dioxide levels and heat. While trees can reduce heat and CO2 levels, complications arise because urban trees tend to live fast and die young.  She shared her research at one of the "Town Talks" put on by Telluride Science and the session was moderated by veteran broadcast journalists Judy Muller and George Lewis

Transcript

Science Straight Up

Season 6, Episode 10

“Trees in the City: Cooling, Carbon and the Complications”

Dr. Lucy Hutyra—Boston University

Moderators: Judy Muller and George Lewis

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JUDY: From Telluride Science, welcome to “Science Straight Up.”

GEORGE: And on this episode…

LUCY: I am a total tree hugger, I really am. I’ve spent my whole life thinking about these things.

JUDY: Summertime in Boston…

CBS BOSTON WEATHERCASTER: Monday, Tuesday and Wednesday, again tracking extreme heat across the region. There’s the range of heat index values, ninety-five to one hundred two.

JUDY: Where they’re baking more than just beans…

WEATHERCASTER: …Hottest day is going to be tomorrow. Well, we ended the week with a heat advisory, beginning next week with a heat advisory.

JUDY: Dr. Lucy Hutyra is the chair of the Department of Earth and Environment at Boston University, and also the recipient of a MacArthur Genius Grant. As our planet warms, she thinks of her city as sort of a petri dish for experimenting on how to cool things down in urban areas and reduce greenhouse gas emissions. The strategies include planting more trees and constructing buildings with so-called cool roofs.

(crowd sounds)

GEORGE: Dr Hutyra came to Telluride to participate in the workshops put on by Telluride Science. About 1200 scientists from all over the world attend these workshops to discuss their cutting-edge research, with one another, also sharing their ideas with the community in a series of so-called “Town Talks.”  Judy and I consider it a privilege to serve as moderators.

JUDY: Dr Lucy Hutyra is an environmental ecologist looking into ways to make urban environments more livable and less globally impactful. Please welcome her to Town Talks (applause)

LUCY: Thank you so much. It's a it's an honor and a pleasure to be here with so many people in this room today. So I've spent most of my career studying trees and how trees impact the atmosphere, how they cycle carbon dioxide, thinking about how they grow, whether it's in the Amazon rain forest or in the middle of cities, and thinking about how they impact air quality, and how they're impacted by air quality. 

GEORGE: Cities cover only about 3 percent of the land area on planet Earth but they account for 75% of global emissions of carbon dioxide. However, that’s not the whole story.

LUCY: Let me add one more statistic. Forests cover about 30% of the global land and take up about 30% of those CO two emissions that come from cities. And so as we walk through this, we're going to think about what is the role that some of those city trees play in the cycling of that carbon dioxide, and think about where those city trees fit into the problem, and what we might be able to do with those trees.

JUDY: As she spoke, she popped up an image of one of the country’s iconic green spaces…Central Park in New York. And she focused on one of the trees planted there.

LUCY: And this urban tree is growing in a very different kind of environment than it would be if it was in a forest. And it's growing very differently than our textbooks would tell us that it grows. So in a forest, trees grow very tall. They invest a lot of their resources in the top of the canopy, trying to collect as much light as they can for photosynthesis to survive and thrive. This tree doesn't have to do that. This tree has few neighbors. It can spread its arms out, its limbs out, rather than only growing up. The soil is very different than what you'd see in a forest. It's not slowly decomposing organic matter in a leaf layer, litter layer that you'd see in a forest. This is a lawn. This is a lawn that's mowed, probably also irrigated and fertilized. So the soil environment is very much dominated by human management of it. And it's not just those growing conditions that are different. This tree has half of its root system is covered by impervious surface, so it doesn't have water that's penetrating. It has runoff coming from the cars and everything else that's driving on that road. It has highly compacted soils. The air that is surrounding this tree is very different than what you'd find in a forest, the ozone is likely higher. The nitrogen oxides are likely higher. The particulate matter that's landing on these trees and on their leaves and sticking to it is higher. It's a very different growing environment.

GEORGE: Trees growing up amid all that urban pollution. You might think their growth would be stunted.  Dr. Hutyra says think again.

LUCY: I was surprised a few years ago when that wasn't what we found. What we found is that these trees, growing in an environment like this are actually thriving. They're thriving in terms of how much carbon dioxide they're taking up, but in many other ways. 

Actually, what we found is these street trees grow four times faster than their country cousins. So if you control for the size of the species, the size of the tree, the species of the tree, they're growing like crazy. The paper that we wrote on this was actually called Live fast Die Young, because they also die young.

JUDY: Urban trees, she says, have a 20-year average lifespan. That’s a problem, because when you grow a tree at a nursery, ship it to the city by truck, dig holes for it with a backhoe, fertilize and maintain it, you’re putting carbon into the atmosphere. The tree will need 20 years to offset that and if it only lives that long, what’s the point? 

GEORGE: The solution lies with finding ways of getting city trees to live longer while reducing the carbon emissions generated by planting and caring for them.  We’ll get back to that in a bit. But Dr Hutyra says there are other benefits to planting trees, most notably in helping cities get cooler. 

LUCY: That process of water leaving the pore of the leaf cools the city, because that water goes from a liquid form inside the leaf to a gas form in the air, and that phase transition from liquid to gas consumes heat. These trees are doing a tremendous amount to change the climate in the city beyond what they're doing to CO two. There's another big mechanism, which is shade. And if you have ever stood under a tree, you know it's cooler under the tree. And so both of these processes are in play. These trees are able to cool our city, but that's just one piece of this story, because the trees are performing this cooling role, but our cities are actually warmer. They're little heat islands, or heat archipelagos within a larger landscape.

(sound—NYC traffic) 

JUDY: And why are cities getting warmer? There are parts of the urban landscape where it’s impractical to plant trees. Infrastructure is a big culprit. Buildings with air conditioning systems that keep things cool inside but throw the heat outside. Asphalt streets absorbing heat from the sun. The same goes for dark-colored roofs. She points out that in Phoenix, Arizona, where people are used to extreme temperatures, a lot of rooftops are light colored to reflect more heat.

LUCY: Phoenix is actually not a very dark city. The surface is very bright, the mean temperature is very high, and they've already adopted some of these cooling strategies. There's more room to go. There are more buildings that could be made more reflective, but they're pretty far along on the spectrum. Boston, on the other hand, is very dark. These are flat roofs that absorb this heat, and inside the buildings also gets hotter.

GEORGE: She and her colleagues recently published the results of an exercise where they hypothetically spent money trying to make Boston cooler.

LUCY: So we took the city of Boston's Parks Department annual budget, $34 million a year, and we spent it, hypothetically spent it, trying to cool the city with a one-time expenditure. and we figured, tried to figure out, where could we have the biggest impact? And to my surprise, it was not trees. In this optimization exercise, only $5 million got invested into trees. Most of the money went to cool roofs.

JUDY: Why? Because the neighborhoods that were hottest, had the most people and the most vulnerable people, simply had no room to plant trees. Re-doing the roofs to reflect sunlight and heat was a better solution.

LUCY: The buildings were close together. The built infrastructure was systematically different with these flat black roofs, where low cost interventions on these roofs could have immediate impact on the temperature, like that day you could change the temperature planting trees can take decades before you start to feel that change.

GEORGE: She says she still remains a big fan of planting trees, although the recipe for offsetting carbon emissions and cooling down cities will vary from place to place.

LUCY: I am a total tree hugger. I really am. I've spent my whole life thinking about these things, but I hope I've lived up to the title, which is thinking about cooling carbon. And some of the complications. Nature based solutions are the best solutions, which they are in a lot of cases. But we really need to think about what problem are we trying to solve? Are we trying to create habitat? Are we trying to cool our cities? Are we trying to offset carbon emissions, or are we just trying to make our city more livable? Because I don't actually want to live in a city that has no trees in it. That would be a little actually, it would be a lot grim to be in a place like that. And so I guess I'll leave you with there are no one size fits all solutions, and when we think about how we try to tackle these problems. We really need to think about it on a city-by-city basis, and look at where that city is along this spectrum, and what are we trying to achieve.

(applause) 

JUDY:  George and I kicked off the question-and-answer part of the program.

GEORGE: As you mentioned, trees have to survive for about 20 years before they're useful as reducers of carbon. In cities, they get peed on by dogs and all sorts of indignities, and that, as you say, means maintenance. And in your words, you don't get headlines for maintenance. Is this partly a PR battle though?

LUCY: (27:12) It's a huge PR battle. It's a huge it's a huge budget battle, because mayors get elected on a really regular cadence, and their constituents want to see that they did something. And maintenance of trees is a hard thing to track. It means you didn't kill the trees. If you're planting trees, you can say, I just planted a million trees in New York City. I'm going to plant another million trees in New York City. There's a different headline associated with that, but most of the ecosystem services come not from those new trees that are planted, but from those trees that you kept alive. And we need to do both, mind you and pretty much every US city is far behind track on the planting rates to be able to just keep the canopy that they have today in the coming decades. So we need to be planting trees, but we need to extend their lives.

JUDY: I know you've worked with cities all over the country, but you told us earlier that Boston is your petri dish, your petri dish, because the Boston politicians trust you, they have come to know, you know what you're talking about, and you know, it's a pretty liberal green place. And under the last administration, there was a lot of emphasis on new tech and meeting green goals, and that's kind of gone. So what's the impact of that change on your work? Having that backing of this is the way to deal with climate change. This is a help. Does it fall on deaf ears? How’s that going?

LUCY: So in Boston, it does not fall on deaf ears. The political will is still there. There's a financial creativity that has to happen, whether it's you know, goody two shoes university professors who have lost large amounts of their federal research dollars, but the commitment is still there, whether it's the city which has lost inflation Reduction Act funds which they had allocated for projects exactly like this. So now they made a commitment to their constituents, but the money that was going to pay for it is gone, and now they're scrambling in their budgets to try to find another way to pay for it. But in the case of Boston, they are not backtracking on commitments. They're trying to figure out how they're going to pay for them.

GEORGE: In Boston and other cities, is there a disparity between affluent white communities and minority areas in terms of the numbers of trees that are on the streets?

LUCY: Well, there certainly is! (LAUGHTER) 

GEORGE: There is. And is that changing?

LUCY: There is, but it's pretty it's pretty complicated, because a lot of that disparity is not about what today's politicians or today's residents did, or who's there now, because it took decades for those trees to get there and establish those canopies. So a lot of it is historical legacies that some of which go back to lending practices and designating some communities as high risk and others as less but it's also what's happening today with is it owners that live there, or is it renters? And do the renters feel empowered to ask the city to plant new trees in front of their house? And what are those calls that the city gets, asking for maintenance and new tree planting and different investments? And if it's immigrant communities, are they going to be calling the city asking for anything right now?

JUDY: Yeah, and you’ve shown us the research shows that urban trees have the benefits for city dwellers. But I just read an article. Maybe this is outside of your scope, but citing studies about the impact on mental health and lowered, lowered incidences of cardiovascular disease in neighborhoods with trees, feelings of community connection. Three separate studies, I found that students in schools with greater tree coverage, especially if they can see them out the windows, perform better academically. You know all this, but I find that astonishing.

LUCY: I think it is absolutely real. You see it in even small things like the Boston Medical Center, which our university runs. They had an experiment where they put a green roof on a portion of the hospital, and patient outcomes were better if they could just look out the window and see it. They didn't even have to be in it, just seeing it had that mental health, uplifting benefit. In you know, around schools, a lot of schools don't have air conditioning, and the Boston area, most schools don't have air conditioning, and so these extended heat waves that are coming earlier, late in the season, that means kids don't go to school because it's too hot in those buildings, they were designed for cold weather and trying to heat them, not for cooling them. And so it's this whole myriad of secondary benefits that the trees provide that as much as that those built solutions, those engineering engineered solutions like air conditioning and cool roofs, those don't get you that that's a single benefit solution. The trees have multiple benefits associated with them.

GEORGE: How important is it to have diversity in the trees that you're planting? I read an article in the Washington Post recently said, Portland, Oregon has a quarter million trees, but most of them are concentrated in four species, like maple, dogwood, cherry, plum, and if there is a disease that affects those trees, you wipe out a large percentage of your cover.

LUCY: Absolutely it's crucial to have that species diversity because of insects, disease and the vulnerability associated with just having one type of tree. But there's a challenge to that, because as much as I said that these trees grow really fast in the city, it's not a random assemblage of species that you have the city, arborists have a preferred planting list of trees. And so how do they choose which trees to plant in the city? Well, they want trees that are going to have a root system that's not going to disrupt sidewalks, that's hopefully not going to break too many pipes. The trees hopefully don't grow too tall, because then they're in the power lines and the maintenance costs get very high on that. They have to be if you're in a really hot place, they have to be tolerant to be in an even hotter place, if you're in a cold place, they have to tolerate salt. So it's not a random set of species. So you need diversity, but you also need to think about planting the right tree in the right place so that it will actually survive.

JUDY: I know we're not in an urban environment here, and when we talk to you about our trees here, you said they have spectator seating. Could you explain just for the eyes, because we're here, in Telluride, right, you look out here now what that means.

LUCY: We've found that trees that are near the edges of forests grow a lot faster. I don't think that's true here. I haven't measured it, but I wouldn't expect it to be true here, because the density, if you look out at these forests here, the density of the trees is so much lower than what you'd see in a northeastern forest, there's space between the trees, and they're all on hillsides, so they have stadium seating, so that extra light, that extra those extra nutrients that get deposited just by having more exposure to the atmosphere that's already happening to all of these trees, because they're just staggered going down the hillside. So they already are benefiting from some of this, so that those edges, those edge effects that we see in the east, I don't think we'd see any of those here.

GEORGE: So, will they live long and prosper because of that? 

LUCY: I think it will help them survive.

GEORGE: There’s a group out at USC, where Judy used to teach journalism, is working on trees in urban environments. And I saw a little video clip that they put out today, and this shows a bunch of young kids from college planting trees all over. LA, but the one thing you didn't mention is that those kids will be middle aged before those trees are much of a benefit to LA…

JUDY: So it requires patience and that’s a hard sell, isn’t it?

LUCY: It is a hard sell, but those sorts of programs do much more than plant trees, right? They're creating a connection between those children that are out there with the shovel, actually digging the hole, trying to put the tree in. Maybe they didn't put it in right. The roots may be compromised, but it's making a connection to nature that will hopefully also last for decades and have a sustained impact, so that there will be a long-term investment into that.

JUDY: So I'm curious about you, because you've devoted your young life to trees, the study of of them. And when did you first get that bug?

LUCY: (laughs) I have amazing parents that were first generation immigrants to the US, and nobody in my family had been to college, and I went to college, and they were like, What do you want to study? And I said, I have no idea. And they, you know, they supported me doing what I wanted. So I chose a major that had really good field trips. (laughter)

LUCY: Sorry, that's the honest answer, and that was where, that's where it started. So I studied forestry.

JUDY: I’m so glad I asked that question.

GEORGE: Beats getting stuck in a lab all the time.

LUCY: That happened later.

JUDY: OK, let’s open this up.

GEORGE: To the audience?

JUDY: To the audience.

WOMAN IN AUDIENCE: I’m just curious about how you said that trees have an average lifespan of 20 years in the cities, but I've definitely been to cities where there's trees that are 200 years old. Old, and, you know, there might be plaques on them or whatever. So why are those trees surviving when others…I don't know if it's an average, or can you just expound on that?

LUCY: Yeah, so it's definitely an average. And you know, those are some of the most valuable trees that you have in the city, those really old ones with the really large canopies that are covering our roads and cooling our roads when you're stuck in traffic, but they're also typically not in a random location, and they probably had larger investments in maintenance to be able to keep them there. Trees are most likely to die when they're planted in cities, or when they're living in cities at two time points in their lives. It has a very U-shaped mortality curve. They die at a very high rate when they're first planted. And so it's actually there's been a huge amount of progress on this front, because cities used to contract planters, groups to plant the trees along roads or wherever it was they were planted, and they had a two year warranty on that planting, so if the tree died within two years of planting, either because it was planted into a natural gas leak, into really terrible soil in the roots were upside down, it would be replaced once Well, the problem is that in the nursery, the trees have what they call stored carbohydrates within their root system that can help sustain them for about two years. And so those trees. If they couldn't establish themselves to be able to acquire those nutrients from the soil that they were planted in, they would often die at 2.1 or 2.5 years. And so luckily, the contracts have changed. It's actually like in the last decade, it's like the most obvious thing that has a huge impact is changing what that contracting structure looks like, and that makes the people planting, the contractors also they do a more careful job. Now, because they're they'll be charged and the large trees, those are often removed when they're providing those the most services, because the hazard is so high, those really large trees, if the limb falls, the damage to infrastructure, can be really meaningful, and especially if there's been a wind storm in the city, you're going to have a run on people that are trying to reduce the hazard to their home so that that large, amazing tree doesn't destroy their home. And so you have this very high mortality due to establishment challenges early on, and then due to the hazard later on. And that comes out to an average of about 20 years. But the services that those trees provide are really skewed towards those large, old trees. So, when I I've had many conversations with the parks commissioner in Boston, and it's like, keep those trees alive. That's the biggest thing you could do to to improve the role of the cities, and providing those nature-based solutions and those services is keeping those large trees alive.

JUDY: So that requires maintenance, and a maintenance crew.

LUCY: That's right.

JUDY Requires payment. 

LUCY: That's right, yeah, that's right. 

JUDY: There we go.

JUDY: Oh, look at this. This is great.

GEORGE: Hands popping up like trees.

MAN IN AUDIENCE: Yes, I had a question. Had they done any studies on using gray water for watering thing and part of this sewage system for cities and that kind of thing?

LUCY: Yes, it's a great way to water the water the trees and some cities are set up for that, and they're especially with golf courses. Oh, I could talk about golf courses, but golf courses often are using that gray water for the irrigation of those sites. And do golf courses cool the city? Yeah, they also consume a tremendous amount of water and nutrients in the form of fertilizers to keep that lush golf course going. So there's a lot of tradeoffs in that space.

JUDY: I can see where you probably come down on it, though, thinking maybe if you had an alternative, 

LUCY: Yeah, I’m not a golfer.

JUDY: I woulda guessed that. (laughter)

GEORGE: Way back in the back…

JUDY: Yes, way back.

MAN IN AUDIENCE: Hi, you're the first person I've heard from Boston. Didn't need an interpreter. So thank you very much. Referring back to George's comment earlier about different species. I grew up in New England, and you're probably a little young to remember this, but there used to be Dutch Elms all over New England, and they're all dead now. I think I have a small recollection of reading something that they're starting to come back. Can you comment? And I know it's a little tangential to your, to your topic, but I thought it'd be interesting.

LUCY: There's elm seedlings that that you you see in the forest you you see them sprouting in the Boston in the center of Boston, in the Commonwealth mall area, there are actually some really beautiful, giant elms that survived the Dutch elm disease. So it wasn't a full decimation, but there's definitely a comeback that's happening. And if you ask me why, I can't answer.

GEORGE: I think that's about all the time we have what? 

JUDY: Yes. Can I just say one thing? 

GEORGE: Sure

JUDY: I just we've loved doing this, and thank you to the science people for doing asking us back. But really, these audiences have been so great. So we want to thank you for coming out and supporting the science talks. (APPLAUSE)

GEORGE: A big thank you to Dr. Lucy Hutyra of Boston University and to Telluride Science for inviting her to speak to the community. 

JUDY: Dr. Hutyra’s appearance was recorded before a live audience at the Telluride Mountain Village Conference Center in Colorado and our audio engineer was Dean Rolley, assisted by Vicky Phelps.

GEORGE: Thank you to our sponsors, Alpine Bank and the Telluride Mountain Village Owners’ Association.

JUDY: Mark Kozak is Executive Director and CEO of Telluride Science and Cindy Fusting is Managing Director and CFO. Sara Friedberg is Lodging and Operations Manager and Annie Carlson is Director of Donor Relations.

GEORGE: If you want to donate to the cause, go to Telluride Science dot O.R.G. That’s also where you can find our podcasts or… look for “Science Straight Up” on your favorite podcast apps. I’m George Lewis…

JUDY: And I’m Judy Muller, inviting you to join us next time on “Science Straight Up.”

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